Laser scanning techniques for writing on a medium sensitive to the laser beam have been disclosed in the prior art. In general, the laser beam is intensity modulated in accordance with information to be printed on a receiving medium, the modulated laser beam being directed to a rotating scanner, or reflector, such as a multifaceted polygon. The rotating scanner in turn causes the modulated laser beam to scan, in sequence, across a sensitive medium located a distance away from the scanner. The information contained in the intensity modulated laser beam can be directly written on the medium if the medium is sensitive to the laser beam, or in an alternative embodiment, the laser beam can selectively discharge a charged insulating surface in accordance with the intensity of the beam. In the alternative embodiment, the degree of charge dissipation corresponds to the information contained in the laser beam. The areas of the medium which are not discharged by the laser beam are subsequently developed, for example, by standard xerographic techniques.
Present day copiers which utilize the xerographic process include a platen upon which the document to be reproduced is placed, the platen being flat or curved. The document is generally flood illuminated and the reflections therefrom are imaged via a copy lens to a charged photoconductive medium to discharge the medium in accordance with the image formed on the document. Although copiers now commercially available are not adapted to utilize laser scanning techniques to scan a document placed on the copier platen, it would be advantageous if such copiers could be modified to incorporate the aforementioned laser scanning techniques. The modified copier would require a system which, inter alia, provides for two-dimensional scanning.
U.S. Pat. No. 3,752,558 discloses a flat field scanning system which utilizes a pulley system with a single motor to move a reflector at one-half the speed of a spaced apart reflector so that the optical distance between the detection station and the document remains constant throughout the scan. However, the patented system does not utilize laser scanning but provides flood type illumination for illuminating a document, such exposure technique having obvious limitations. The use of laser flux for illumination provides significant advantages radiometrically over conventional white light illumination systems. The particular requirements/problems involved when using a laser scanning system are not present when flood type illumination is utilized. For example, when a two-dimensional laser scanning system is utilized it is required that the scanning of the document be directly synchronized with the movement of the scanning laser beam. Further, the utilization of the laser type scanning system requires that compensation be provided in order that the variation in the trajectory of the laser beam at the platen be minimized. Additionally, unless the scanner is illuminated by the laser beam at a proper angle of incidence, the beam will be non-symmetrically reflected by the scanner giving rise to non-symmetrical deflections in one of the scan directions upon each side of the center of the scan line.